This invention generally relates to thermoformable barrier films and to vacuum skin packages which can be made therefrom. More particularly, the invention relates to vacuum skin packaging utilizing multilayer gas barrier films comprising a peelable gas-impermeable (i.e., barrier) film adhered to a gas-permeable film, and a peel mechanism for easily removing the gas-impermeable film from the gas-permeable film.
Skin packaging can be classified as a vacuum forming process for thermoformable polymeric films. The product on a supporting member serves as the mold for the thermoformable film which is formed about the product by means of differential air pressure. The term "vacuum skin packaging" (hereinafter "VSP") as used herein indicates that the product is packaged under vacuum and the space containing the product is evacuated from gases. It is therefore desirable that the film formed around the product and for the support member to each present a barrier to oxygen, air, and other gases detrimental to the shelf or storage life of a product such as a food product.
Skin packaging is described in many references, including French Patent No. 1,258,357, French Patent No. 1,286,018, Australian Patent No. 3,491,504, U.S. Pat. No. RE 30,009, U.S. Pat. No. 3,574,642, U.S. Pat. No. 3,681,092, U.S. Pat. No. 3,713,849, U.S. Pat. No. 4,055,672, U.S. Pat. No. 4,889,731, and U.S. Pat. No. 5,346,735. The disclosures of each of the foregoing references is hereby incorporated herein by reference. Typically, skin packaging provides just a barrier film that upon removal from the package leaves the product exposed to atmosphere, which is sufficient for applications such as bulk meat cutting and repackaging. It is desirable in other applications to provide a composite packaging film comprising both a gas-permeable (i.e., non-barrier) film and a peelable gas-impermeable film so that upon removal of the peelable gas-impermeable film the product is still selectively protected by the gas-permeable film. This permits a product such as a fresh red meat to be protected by the gas-permeable film but allows the meat to "bloom" from a purplish color to a customer-preferred reddish color upon exposure to oxygen.
One problem with all such peelable composite films is the difficulty of initiating the peel. Although, generally, the gas-impermeable film will peel easily from the gas-permeable film after initiation of the peel, actually beginning the peeling process can be difficult. Without some peel initiation mechanism in place, the person attempting to peel the gas-impermeable film from the gas-permeable film must flick at the package edge with his thumb or fingers and, often, packages are damaged when further measures become necessary. Thus, providing an acceptable peel initiation mechanism has become an important aspect of designing any package which employs a peelable composite film.
One example of a prior art peel initiation mechanism is set forth in U.S. Pat. No. 5,346,735, referenced above. During the process of making a vacuum skin package, a tab is placed on, but not adhered to, the lower support web, extending along one edge. When the upper composite film web is formed around the product contained on the lower web, it welds to the upper surface of the tab. When the tab is pulled, the gas-permeable film, which is more strongly bonded to the lower web than to the gas-impermeable film, ruptures at or near the tab and the gas-impermeable film is peeled therefrom. However, during the package forming process it is difficult to automate the precise placement of the tab and after peeling of the gas-impermeable film from the gas-permeable film the torn edge of the gas-permeable film is noticeable, detracting from the desired neat appearance of the package.
An alternative prior art peel initiation mechanism is disclosed in U.S. Pat. No. 4,889,731, also referenced above. A label having a non-sealable upper surface is adhered to the lower web, extending along one edge. When the upper composite film web is formed around the product contained on the lower web, it does not adhere to the upper surface of the label. A series of perforations are formed through the upper and lower webs adjacent to the label. When the upper web is pulled from the label the perforations provide a point of rupture for the gas-permeable film which is more strongly bonded to the lower web than to the gas-impermeable film, and the gas-impermeable film is peeled therefrom. However, as above, during the package forming process it is difficult to automate the precise placement of the label and after peeling of the gas-impermeable film from the gas-permeable film the torn edge of the gas-permeable film is noticeable, detracting from the desired neat appearance of the package.
It is therefore desirable to provide a peel mechanism for use with peelable composite packaging films without these disadvantages.